Method of adsorption of cationic and anionic polymers on the surface of particles and paper or nonwoven products containing such particles

ABSTRACT

The present invention provides a method of producing a particle or group of particles intended for use in making paper- and/or nonwoven products and having a coating of at least two, preferably at least three, thin layers of alternating cationic and anionic polymers located outside each other, in which the particle or group of particles is treated in consecutive steps with solutions of the alternating cationic and anionic polymers. The amount of the respective polymer to be added in each step is controlled by charge measurements of the treatment solution or a liquid containing the particles or group of particles and the polymer solution, after the treatment in each step in order to determine that substantially all polymer is adsorbed to the particle surface. The present invention also relates to a paper- or nonwoven product containing fibers and/or fillers produced according to a method of the invention. The present invention also encompasses paper products containing increased amounts of wet strength agent and tissue paper having improved wet strength.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of International Application No. PCT/SE01/00612, which was published in English on Oct. 18, 2001, the entire contents of which are hereby incorporated by reference in their entirety. In addition, the application claims the priority of SE 0001268-2, filed Apr. 6, 2000 in Sweden, the entire contents of which are hereby incorporated by reference in their entirety.

FIELD OF THE INVENTION

[0002] The present invention refers to a method of producing a particle or group of particles intended for use in making paper- and/or nonwoven products having a coating of at least two, preferably at least three, outside each other located thin layers of cationic and anionic polymers, at which the particle or group of particles is treated in consecutive steps with solutions of the cationic and anionic polymers. It also refers to a paper or nonwoven product containing such particles or groups of particles. It further refers to paper products containing increased amounts of wet strength agent and to tissue paper having improved wet strength.

BACKGROUND OF THE INVENTION

[0003] The increased use of recovered fibers in paper production and the use of components with poorer bonding properties, such as mineral fillers, have increased the need for more effective dry and wet strength agents in the paper. Traditionally two different methods have been used for adding strength improving chemicals to the paper, viz. by adding chemicals at the wet end of the paper process or by surface application by means of a size press. Wet end addition is usually more effective than surface application counted per kg utilized product. In order to maintain the addition made in the wet end in the paper sheet, wet end chemicals are mainly exclusively cationic, and for making them less sensitive to dissolved and colloidal materials and the increased concentration of electrolytes caused by the increased closing of the systems, their cationic charge is usually increased. This leads in turn to a decreased saturation adsorption of the additive chemicals to the fibers, which leads to a reduced maximum effect of the additive chemicals. This involves that there is a need both for new methods of applying strength-improving chemicals to the paper, and new chemical systems.

[0004] Additionally, there is an increased need for improving the opacity of the finished paper. Because most frequently used strength agents today contribute negatively to the opacity, the need for new methods of developing strength in the paper is further reinforced.

[0005] Such a way would be to utilize size presses to a higher extent, but this would however lead to large reductions of the manufacturing capacity and the production economy because the paper has to be dried once further depending on the rewet it is exerted to in the size press.

[0006] Accordingly, there is a great need for new ways of treating fibers and other particles contained in the paper, such as filler particles, in the wet end of the paper machine.

[0007] It is known to build up thin multilayers of electro active polymers on an electrostatically charged substrate for use in optics, such as sensors, friction reduction etc. This is described in for example in Thin Solid Films, 210/211 (1992) 831-835 and in Thin Solid Films, 244 (1994) 806-809. The substrate is herewith immersed alternatingly in diluted solutions of a polycation with an intermediate rinsing in order to remove rests of the previous polyion which is not bonded to the substrate. The thickness of each deposited layer is described to be between 5-20 Å. There is no indication that the treated substrates could be particles, such as fibers or filler particles.

[0008] In U.S. Pat. No. 5,338,407 there is disclosed a method for improving the dry strength properties of paper, at which an anionic carboxy methyl guar or carboxy methyl hydroxy ethyl guar and a cationic guar is added to the furnish. These two components are either added in mixture or separately. There is no indication that the treatment is made under such conditions that a double layer is built up on the fibers with one component in one layer and the other component in the other layer.

[0009] In the U.S. Pat. Nos. 5,507,914 and 5,185,062 there are disclosed methods for improving the dewatering properties and the retention of paper by adding anionic and cationic polymers to the pulp. There is no indication that the treatment takes place under such conditions that a double- or multilayer is created on the pulp fibers with the anionic component in one layer and the cationic component in the other layer.

[0010] Dual surface treatment of filler particles with anionic and cationic polymers is disclosed in EP-A-0 850 879, WO 95/32335, U.S. Pat. No. 4,495,245 and U.S. Pat. No. 4,925,530. There is no indication that the treatment takes place under such controlled conditions that a double- or multilayer is created on the pulp fibers with the anionic component in one layer and the cationic component in the other layer.

[0011] In the international patent application PCT/SE99/02149 there is disclosed a method as stated in the introduction above and according to which particles or groups of particles are produced which have a coating of at least two, preferably at least three, outside each other located thin layers of interacting polymers. The particles or groups of particles are treated in consecutive steps with solutions of the interacting polymers, and the respective polymer is added only in such an amount in each step that substantially all polymer is adsorbed to the particle surface. This is accomplished by removing excess of the previous polymer between each treatment step alternatively by adding the respective polymer only in such an amount in each step that substantially all polymer is adsorbed to the particle surface.

SUMMARY OF THE INVENTION

[0012] The object of the present invention is to provide a method according to above that offers a way to ensure that each polymer for forming the respective layer on the surface of the particles or group of particles is added only in such an amount in each step so that substantially all polymer is adsorbed to the particle surface. According to the invention, this has been accomplished by the fact that the amount of the respective polymer to be added in each step is controlled by electric charge measurements of the treatment solution or a liquid containing the particles or group of particles and the polymer solution, after the treatment in each step in order to determine that substantially all polymer is adsorbed to the particle surface.

[0013] In order to determine the amount of cationic/anionic polymer adhered to the particle surface, the Z-potential of the particles or groups of particles may be measured.

[0014] The particles or groups of particles may be of various types. However, fibers, e.g., cellulosic fibers, regenerated fibers, and different types of synthetic fibers and filler particles are preferred embodiments.

[0015] The interacting polymers are preferably alternating cationic and anionic polyelectrolytes, but they may also be so-called zwitter ions.

[0016] According to one embodiment of the invention, the particles are cellulose fibers for papermaking and at least one of the polymers is a strength additive such as a wet and/or dry strength agent.

[0017] Another embodiment of the invention comprises a paper- or nonwoven product that contains fibers and/or filler particles produced by the method described above. The term “paper” used herein refers to all types of paper, such as tissue paper, graphical paper, linerboard, wiping material, etc. The nonwoven material could be of various types.

[0018] Another embodiment of the invention comprises paper products containing increased amounts of wet strength agent and to tissue paper having improved wet strength.

BRIEF DESCRIPTION OF THE FIGURES

[0019]FIG. 1 shows the results of Z-potential measurements of cellulose fibers treated in consecutive steps with cationic and anionic polymers in the form of PAE (polyamino-amide-epichlorhydrine) and CMC (carboxy methyl cellulose).

[0020]FIG. 2 shows the result of charge measurements of the colloidal phase before and after washing the fibers in the trial of FIG. 1.

[0021]FIG. 3 shows the wet strength tensile index of 30 gsm paper sheets made from the treated fibers vs. adsorbed amount of PAE.

[0022]FIG. 4 shows Z-potential of the fibers after addition of PAE/CMC/PAE.

[0023]FIG. 5 shows the charge of the colloidal phase from PCD measurements after addition of PAE/CMC/PAE.

[0024]FIG. 6 shows the dry tensile strength index vs. adsorbed amount of PAE.

[0025]FIG. 7 shows the wet tensile strength index vs. adsorbed amount of PAE.

[0026]FIG. 8 shows relative wet strength vs. adsorbed amount of PAE.

DETAILED DESCRIPTION OF THE INVENTION

[0027] In one embodiment of the present invention, particles or groups of particles, e.g., fibers or filler particles, are treated with alternating cationic and anionic polymers in order to build up thin multilayers of the interacting polymers on the particle surface.

[0028] The particles are treated in consecutive steps with solutions of the alternating cationic and anionic polymers, at which the treatment time for each step is sufficient for forming a layer of the desired molecular thickness. For particles or groups of particles having an anionic surface, e.g., cellulosic fibers, the first layer should be a cationic polymer, and vice versa. By adding the polymers in consecutive steps and letting them form several layers on the particle surface, it is possible to adsorb higher amounts of polymer to the particle surface than is possible when adding them in one step intermixed forming only one layer.

[0029] The addition is controlled in such a way that substantially no excess amount of the respective polymer is added in each step, so that substantially all polymer is adsorbed to the particle surface. This is made by measuring the electric charge of the treatment solution or the liquid in which the treated particles or groups of particles are contained. After having allowed the polymer to adsorb to the particle surface a certain period of time the electric charge of the solution should be close to zero. In one embodiment, the charge measurements are made with streaming potential measurement, e.g., with a PCD instrument (Particle Charge Detector).

[0030] In order to determine the amount of cationic/anionic polymer adhered to the particle surface the Z-potential is measured according to a preferred embodiment of the method described below.

[0031] The method according to the invention for building the desired multilayers is based on electrostatic attraction between oppositely charged polyelectrolytes. By treating the particles in consecutive steps with a solution containing polyions of opposite charge and permit these spontaneously to adsorb to the particle surface, multilayers of the stated kind are built up. In principle, all types of polyelectrolytes may be used.

[0032] According to one embodiment, the method is used for adsorbing strength additives to cellulosic fibers used for papermaking. Because the cellulose fiber has an anionic surface, the first polymer to be adsorbed is a cationic polymer. This may be, e.g., polyaminoamide-epichlorhydrine (PAE) or glyoxylated polyacryl amide (G-PAM). This layer will make the fiber surface cationically charged. In a next step an anionic polymer, e g CMC (carboxy methyl cellulose), is added. The fiber surface will then turn anionic again. Then the next layer of cationic polymer can be added and so on. Through such a treatment of cellulose fibers, higher amounts of strength additives, e.g., PAE or G-PAM, can be adsorbed to the fiber surface than is possible with conventional techniques, which will result in improved strength properties of the paper produced.

[0033] It is of course also possible to make new types of surface modifications to particles or groups of particles through consecutive adsorption of thin layers of interacting polymers according to the invention. For example, by treating fibers with consecutive layers of hydrophobic, charged polyelectrolytes it would be possible, e.g., to develop new types of hydrophobizing chemicals for the hydrophobization of paper. It would also be possible to build up “intelligent” surface layers on fibers, which alter the properties with temperature, pH, salt content etc.

[0034] Other embodiments include ion-exchanging fibers where “membranes” with ion-exchanging properties are provided on the fiber surface, wet strength agents where the added polymers are reactive with the fibers and with each other, in order to provide permanent bonds between the fibers and for the production of highly swelling surface layers, where the added chemicals form swollen gel structures on the fiber surface for use in absorbent hygiene products. Additional embodiments are new types of fibers for printing paper, where the adsorbed polymers change colour when they are exerted to an electric, magnetic or electromagnetic field. Such polymers are available today.

[0035] The fibers that are treated with a method according to the invention may be of various types, including natural as well as synthetic fibers. In a preferred embodiment, mainly cellulosic fiber are concerned. However it would be possible to treat synthetic fibers, e.g., for giving them a more hydrophilic surface.

[0036] In other embodiments, groups or agglomerates of fibers or particles can be treated according to the method.

[0037] Examples of suitable anionic and cationic polyelectrolytes which may be used in embodiments of a method according to the invention are given below. Anionic polyelectrolytes: Anionic starch with different degrees of substitution, anionic guar, polystyrene sulfonate, carboxy methyl cellulose with different degrees of substitution, anionic galactoglucomannan, polyphosphoric acid, polymethacrylic acid, polyvinyl sulphate, alginate, copolymers of acryl amide and acrylic acid or 2-acrylic amide-2-alkylpropane sulphonic acid.

[0038] Cationic polyelectrolytes: Cationic galactoglucomannan, cationic guar, cationic starch, polyvinyl amine, polyvinyl pyridine and its N-alkyl derivatives, polyvinyl pyrrolidone, chitosan, alginate, modified polyacryl amides, polydiallyl dialkyl, cationic amide amines, condensation products between dicyane diamides, formaldehyde and an ammonium salt, reaction products between epichlorhydrine, polyepichlorhydrine and ammonia, primary and secondary amines, polymers formed by reaction between ditertiary amines or secondary amines or dihaloalkanes, polyethylene imines and polymers formed by polymerisation of -(dialkylaminoalkyl)acrylic amide monomers.

EXAMPLES

[0039] The polymers used during the tests are listed in Table 1 below. TABLE 1 Viscosity Name Polymer Supplier D.S.* (mPas) Kenores Polyaminoamide- EKA 1440 epichlorohydrin, (PAE) Chemicals Parez Glyoxalated polyacrylamide, Cytec 631 NC (G-PAM) Industries Cekol Carboxymethyl cellulose, Metsä 0.78 7200 50000G (CMC) Speciality (1% conc.) Fibers Finnfix Carboxymethyl cellulose, Metsä 0.4- 100-700 BW (CMC) Speciality 0.6 (4% conc.) Fibers

[0040] The PAE and G-PAM were diluted in deionized water to an active content of 10 g/l before use. The different CMC's were dissolved in deionized water by dispersion using a hand mixer, to a suitable concentration between 5 and 10 g/l depending on the viscosity.

[0041] The pulp used was a dried fully bleached TCF, Celeste 85, from SCA Östrand. The pulp was beaten to 25° SR and was diluted with tap water to a concentration of 3 g/l. The pH during the trials was 7.5 and the conductivity was set to 1200 μS/cm using NaCl.

[0042] Three, four and five layers of alternating PAE and CMC were made. The addition of the additives were controlled by measuring the electric charge with Z-potential and PCD instruments. In some tests G-PAM was used instead of or together with PAE. The addition sequence is listed in Table 2 below. TABLE 2 Addition (mg/g) G- G- Trial PAE CMC PAE CMC PAE PAM CMC PAE PAM 1 20  20  20 20  20 2 15  7 30 4 15 3 15  4 15 4 15  4 30 5 7 2  7 6 7 2 10 7 7 2 15 8 5 1  5 1  5 9 2  7 2  7 10  7 2 7 11  7 2 7 12  7 2 7

[0043] Adsorption time for the additives was 10 minutes.

[0044] Furnish Preparation

[0045] In trial 1 the PAE and CMC additions were considerably overdosed, 20 mg/g pulp. The pulp was then dewatered, washed with deionized water and dewatered again between each step in order to remove the excess of PAE and CMC. This was repeated until five layers were made, starting and ending with PAE, see Table 2 above.

[0046] In trials 2-12 the PAE and CMC additions were successively optimized to more realistic levels. Both three, four and five layers were made, see Table 2. The addition of the additives were made with no washing step in between, always controlled by measuring the charge with Z-potential and PCD instruments. Three trials using G-PAM instead of or together with PAE were performed in order to evaluate another wet strength additive using the multilayering technique.

[0047] Sheet Preparation

[0048] 30 g/cm² sheets were formed in a dynamic sheet former. The sheets were pressed between blotters and then dried under restrained conditions. Two sheets of each sample point were formed and pressed to two different densities, ˜350 and ˜450 kg/m³. Finally, the sheets were cured for 10 minutes at 105° C. The tensile strength was then interpolated to a density of 400 kg/m².

[0049] Charge Determinations

[0050] Fiber Charge Determination Using a Z-Potential Instrument

[0051] The Z-potential of the fibers was measured with a streaming potential instrument (Magendans SZ2, supplied by Mütek) [(Penniman, J. G., Comparison of pulp pad streaming potential measurement and mobility measurement. Tappi Journal, 1992 75 111-115 and Jaycock, M. J.; Assumptions made in the measurement of zeta-potential by streaming current/potential detectors. Paper Technology, 1995 36 35-38.19, 20; Barron, W., et al., The streaming current detector: a comparison with conventional electrokinetic techniques. Colloids and Surfaces, 1994 88 129-139; Sanders, N. D. and J. H. Schaefer, Comparing papermaking wet-end charge-measuring techniques in kraft and groundwood systems. Tappi Journal, 1995 78 142-150).

[0052] The potential is measured between two electrodes, one screen electrode placed close to the fiber pad, which is formed when pumping it against a screen, and the second electrode is a ring electrode situated on the lower part of the fiber pad. Conductivity is then measured between these two electrodes and a calculated value of the Z-potential (z) is presented.

[0053] Colloidal Charge Determination Using a PCD

[0054] A PCD 03 (Particle Charge Detector) supplied by Mütek measures a voltage difference induced by a moving charged medium, e.g., colloidal substances in a white water. High molecular mass polymers and colloidal substances attach to the Teflon surfaces of the equipment. The oscillating piston moves and induces a potential difference that is detected (Jaycock, M. J., Assumptions made in the measurement of zeta-potential by streaming current/potential detectors. Paper Technology, 1995 36 35-38 ; Barron, W., et al., The streaming current detector: a comparison with conventional electrokinetic techniques. Colloids and Surfaces, 1994 88 129-139 and Sanders, N. D. and J. H. Schaefer, Comparing papermaking wet-end charge-measuring techniques in kraft and groundwood systems. Tappi Journal, 1995 78 142-150.20-22).

[0055] The results are presented, as the charge in μeq/l, calculated from the amount of reference polymer needed to titrate to zero charge. With PCD, it is the charge of the colloidal phase that is measured.

[0056] Tensile Strength Evaluation

[0057] Strength evaluation of the sheets was performed according to the standard methods SCAN P 44:81 for dry tensile strength. For wet tensile strength the test pieces were tested according to SCAN P 58:86. A deviation from these methods was that the width of the test pieces was 15 mm. Soaking time of the test pieces before tensile strength test was 15 s. The tensile strength results are presented as geometrical mean values of machine and cross direction, $\frac{\sqrt{{MD} \cdot {CD}}}{grammage}N\quad m\text{/}{g.}$

[0058] Analysis of Strength Additives in Sheets

[0059] Total Nitrogen Content

[0060] PAE and G-PAM adsorption in the sheets was analysed by measuring the total nitrogen content in the sheets. The method is based on flash combustion and is called Dumas Total Nitrogen Analysis and the measuring instrument used is Carlo Erba Instrument NA 1500 supplied by CE Termo Quest. A manual is supplied together with the instrument.

[0061] Ion Exclusion Chromatography

[0062] In some tests PAE and G-PAM adsorption in the sheets was analysed by ion exclusion chromatography.

[0063] 1 g of paper sample is hydrolysed with 1.0 M NaOH. The hydrolysis is done at 100° C. for 24 hours. The PAAE resin is then hydrolysed to DETA and Adipate (see formula below).

[0064] The solution is neutralised with an ion exchanger and the resulting adipic acid is analysed with ion exclusion chromatography. The wet strength resin is treated and analysed in the same way to calculate the amount of adipic acid in the actual resin. This result is then used to calculate the amount of PAE resin in paper.

[0065] Ion exclusion chromatography is mainly used for analysis of weak inorganic and organic acids. The chromatographic column is packed with a stationary phase consisting of a sulfonated polystyrene/divinylbensene based cation exchanger. Depending on the degree of crosslinking of the polystyrene/divinylbensen resin, different organic acids may diffuse into the stationary phase to a greater or lesser degree. This mechanism together with ion exchange is used for chromatographic separation of organic acids in solution.

[0066] Suppressed conductivity is used for detection. The equipment used for the analysis is described below: Columns: 2 × Dionex ICE-AS1 columns in series Suppressor: Dionex AMMS-ICE Detector Dionex PED (Conductivity mode) Eluent: 1.0 mM Heptafluorobutyric acid; 0.8 ml/min

[0067] The working range for the method is 0.01-1.0% in paper (calculated as dry PAE resin) and the relative standard deviation for a paper sample with 0.3% PAE (dry resin) is 3.8%.

[0068] Trial 1

[0069] In trial 1, both PAE and CMC were added in excess according to the addition sequence of Table 2. This was done to ensure saturated fibers. During this trial Z-potential and PCD measurements were performed to control both the adsorption of polymers and the desorption of polymers in the intermediate washing step.

[0070]FIG. 1 shows the Z-potential of the fibers during trial 1, before and after washing of the fibers. As can be seen, the Z-potential is not largely influenced by washing the fibers. A small decrease in the observed Z-potential can be detected when washing the PAE treated fibers and a small increase when washing the CMC treated fibers, which presumedly is due to the additive desorption during the washing step.

[0071]FIG. 2 shows the results of charge measurements of the colloidal phase (PCD measurements) during trial 1, before and after washing of the fibers. FIG. 2 shows that when adding PAE/CMC in excess a large amount of the added polymer stays in the colloidal phase instead of adsorbing to the fibers. In the washing step the excess is removed.

[0072]FIG. 3 shows the wet tensile index of the sheets versus the adsorbed amount of PAE. When five layers were made the wet tensile index levels out, even though the adsorption of PAE increases. The explanation to this is not clear. Anyway it is clear that by a multilayering technique it is possible to increase the amount of PAE to the fibers. Using a single addition point, about 11 mg/g of PAE is possible to adsorb on the fibers. Using three layers, more than 20 mg/g PAE is possible to adsorb with a resulting increase in wet tensile strength of 50%.

[0073] Dry tensile strength index show a similar trend as the wet tensile strength index but, as expected, the increase in dry strength is not as high as the increase in wet strength.

[0074] Trials 2-12

[0075] In these trials, different amounts of PAE/CMC were added and the Z-potential and colloid phase charge were measured. The results from some of these measurements are shown in FIGS. 4 and 5. Corresponding figures for addition in four and five layers show the same trends, i.e., increases with PAE addition and decreases with CMC addition.

[0076] As shown in FIG. 5, the charge in the colloidal phase is balancing around zero charge, indicating that the adsorption of PAE/CMC on the fibers is almost total, i.e., not much of the additives end up in the water phase. The deviation from zero charge should preferably not exceed ±5 μeq/l, more preferably not exceed ±2 μeq/l.

[0077] In FIG. 6 there is shown the dry tensile strength index versus adsorbed amount of PAE, at which, e.g., “7/2/7-10-15” means 7 mg/g PAE, 2 mg/g CMC, then 7, 10, 15 mg/g PAE in the third layer. In the trials the dry tensile strength index reached its highest level at relatively low adsorbed amounts of PAE. At an adsorbed amount of approximately 5 mg/g the strength is levelling out.

[0078] In FIG. 7 there is shown the wet tensile strength index versus adsorbed amount of PAE.

[0079]FIG. 8 shows the relative wet strength versus adsorbed amount of PAE. FIGS. 7 and 8 show that wet tensile strength index and the relative wet strength level out, but it seems like the highest level is not fully reached. As mentioned above, the dry tensile index start to level out at 5 mg/g adsorbed amount of PAE. The wet tensile strength index levels out but at higher levels of adsorbed amount of PAE. A maximum relative wet strength of 40% is reached.

[0080] However, the absolute values are also very interesting. A wet tensile index of almost 30 Nm/g is achieved using the multilayer technique. Normally 65 g/m² lab sheets end up at approximately 10 Nm/g.

[0081] When studying FIGS. 6-8, it seems like three layers is enough to reach maximum strength with the concept used. With three layers, it was possible to almost reach the highest strength levels. However, this may differ for other types of additives and/or other types of particles than cellulose fibers.

[0082] For the trials using G-PAM instead of or together with PAE, basically the same effect was obtained as for PAE. The wet tensile strength index was slightly lower when using G-PAM compared to PAE, while in some cases a higher dry strength was obtained. The order of addition when using both PAE and G-PAM seemed to effect the result, e.g., the adsorption of strength additive was higher when starting with PAE than with G-PAM.

[0083] The trials show that charge measurements using PCD and Z-potential instruments provide good control of polymer addition. The multilayering technique gives an increased amount of additives that are adsorbed to the fibers, which helps to give, e.g., an increased strength up to a certain level.

[0084] In a full scale continuous process, such as a papermaking process, the amount of polymer to be added is preferably controlled and determined by Z-potential and PCD measurements after each addition of polymer in each step during the starting up of the process. These amounts are then used in the process. The Z-potential and PCD measurements are during the run of the process preferably performed only after the headbox. Addition of the first polymer is, e.g., made in the pulper, and the other polymers are then added at different steps in the wet end of the paper machine.

[0085] In the examples above, only the addition of strength additives to cellulose fibers for papermaking are described. It is, however, to be understood that the invention may be applied for consecutive adsorption of thin layers of optional types of alternating cationic and anionic polymers on the surface of fibers or other types of particles or groups of particles in order to build up thin multilayers of the interacting polymers on the particle surface.

[0086] By adding the polymers in consecutive steps and letting them form several layers on the particle surface, it is possible to adsorb higher amounts of polymer to the particle surface than is possible when adding them in one step forming only one layer. With regard to wet strength agents such as PAE and G-PAM, it is possible in a preferred embodiment of a method of the invention to produce paper and nonwoven products containing at least 1.5, preferably at least 1.7, more preferably at least 2.0, even more preferably at least 2.2 and most preferably at least 2.5% by weight or more of a wet-strength agent. These values refer to the amount of wet-strength agent adhering to the fibers and measured according to the total nitrogen method disclosed above. In some of the laboratory trials, up to 3.8% by weight wet-strength agent adhered to the fibers (FIGS. 7 and 8).

[0087] This also means that, in a preferred embodiment of a method according to the invention, it is possible to produce tissue paper based on cellulose fibers with no admixture of other types of fibers, such as synthetic reinforcing fibers, having a wet tensile index of at least 6.5, preferably at least 7.0 and more preferably at least 7.5 Nm/g. The term “tissue paper” in this respect does not include materials exerted to hydroentangling. These values refer to finished tissue products produced on a full-scale tissue machine. FIG. 7 shows a laboratory made products in which you normally achieve higher strength values. In finished tissue products produced on full-scale tissue machines and with subsequent treatments like creping, converting, etc., the strength values are decreased.

[0088] This is shown in the results presented in Table 3 below showing the results of strength measurements on two different wet-strong tissue papers. No. 1 is a tissue paper used as wiping material sold by SCA Hygiene Products AB under the trademark “M-Tork” and having the following pulp composition: 33% by weight CTMP and 67% by weight softwood kraft pulp (TCF). It contains about 0.7% by weight PAE. No. 2 is a paper produced from the same type of pulps as No. 1 and where the cellulose fibers were treated in consecutive steps according to a preferred embodiment of the invention with two layers PAE, one layer G-PAM and two layers CMC. It is seen from these results that tissue paper no. 2 showed improved strength properties. It is further noted that the papers tested contained a mixture of CTMP and softwood kraft pulp. For papers containing higher amounts of or only containing softwood kraft pulp even higher strength values would be expected. TABLE 3 Sample 1 (ref.) 2 (invention) Grammage g/m² 24 21 Thickness 2 kPa μm 154 162 Bulk 2 kPa cm³/g 6.4 7.7 Tensile strength MD, dry N/m 242 438 Tensile strength CD, dry N/m 223 340 Tensile index ✓MDCD dry Nm/g 10 18 Stretch MD % 28 37 Stretch CD % 4.9 4.6 Stretch ✓MDCD % 11.7 13.1 Work to rupture MD J/m² 39 78 Work to rupture CD J/m² 7.983 13.63 Work to rupture index ✓MDCD J/g 0.7 1.6 Tensile strength MD, water N/m 102.9 206.8 Tensile strength CD, water N/m 60.8 125.8 Tensile index ✓MDCD water Nm/g 3 8 Relative strength water % 34 42

[0089] Table 4 shows the results of measurements for determining the amount of wet strength agent in the form of PAE in some commercially available tissue products and in a tissue paper made with the method according to the invention. Sample A is a tissue paper made according to the invention corresponding to the one tested as No. 2 in Table 3. Sample B is a tissue paper produced by Fort James and sold under the trade name “Lotus Profes”. Sample C is a tissue paper produced by Procter & Gamble and sold under the trade name “Bounty”. Sample D is a tissue paper produced by Metsä Särla and sold under the trade name “Katrin Cleany”.

[0090] The amount of PAE in the different tissue papers were measured by the ion exclusion chromatography method described above and gives the amount of PAE adsorbed to the fibers. It is to be noted that normally the amount of PAE or other wet strength agent added to the furnish is given as % of the wet strength agent solution added per weight fibers. Wet strength agents are sold as solutions containing between about 6 and 25% of the active component. The amount of wet strength agent refers to the amount of the active component adhered to the fibers. TABLE 4 Sample A (invention) B (ref.) C (ref.) D (ref.) Amount PAE 2 0.45 1.2 0.7 (% by weight)

[0091] As is seen from these results, paper A produced according to the invention contained considerably higher amounts of PAE than the commercial wet-strong tissue products tested.

[0092] Although only preferred embodiments are specifically illustrated and described herein, it will be appreciated that many modifications and variations of the present invention are possible in light of the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention. 

What is claimed is:
 1. A method of producing a particle or group of particles for paper or nonwoven products, the particle or group of particles comprising a coating of at least two thin layers of alternating cationic and anionic polymers located outside each other, comprising: treating the particle or group of particles in consecutive steps with solutions of the alternating cationic and anionic polymers wherein the amount of the respective polymer to be added in each step is controlled by charge measurements of each of the polymer solutions or a liquid containing the particle or group of particles and each of the polymer solutions during or after the treatment in each step in order to determine that substantially all polymer is adsorbed to the particle surface in order to produce a particle or group of particles having a coating of at least two thin layers of alternating cationic and anionic polymers.
 2. A method of claim 1, further comprising measuring the Z-potential of the particles or groups of particles during and/or after the treatment in each step in order to determine the amount of cationic/anionic polymer adsorbed to the particle surface.
 3. A method of claim 1, wherein the particle is a fiber.
 4. A method of claim 3, wherein the fiber is a cellulosic fiber.
 5. A method of claim 3, wherein the fiber is a synthetic or regenerated fiber.
 6. A method of claim 1, wherein the particle is a filler particle, a coating particle, or other type of paper-making particle.
 7. A method of claim 1, wherein at least one of the anionic or cationic polymers is a strength additive used in papermaking.
 8. A method of claim 1, wherein the process for consecutive absorption of thin layers of interacting polymers on the surface of particles or groups of particles is continuous and wherein the amount of the respective polymer to be added in each step is determined at the starting-up of the process.
 9. A paper or nonwoven product comprising fibers, filler particles, or other particles produced according to the method of claim
 1. 10. A paper or nonwoven product comprising at least three thin layers of alternating cationic and anionic polymers located outside each other and at least 1.5% by weight of a wet-strength agent.
 11. A paper or nonwoven product of claim 10, wherein the wet-strength agent is a cationic polyelectrolyte.
 12. A tissue paper based on wood pulp cellulose with no admixture of other types of fibers that has a wet tensile index of at least 6.5 Nm/g.
 13. A tissue paper of claim 12, wherein said tissue paper contains at least three thin layers of alternating cationic and anionic polymers located outside each other.
 14. A method of claim 1, wherein the coating contains at least three thin layers of alternating cationic and anionic polymers.
 15. A method of claim 7, wherein the strength additive is a dry-strength and/or a wet-strength agent.
 16. A paper or nonwoven product of claim 10, wherein the paper or nonwoven product contains at least 1.7% by weight of a wet-strength agent.
 17. A paper or nonwoven product of claim 10, wherein the paper or nonwoven product contains at least 2.0% by weight of a wet-strength agent.
 18. A paper or nonwoven product of claim 10, wherein the paper or nonwoven product contains at least 2.2% by weight of a wet-strength agent.
 19. A paper or nonwoven product of claim 10, wherein the paper or nonwoven product contains at least 2.5 % by weight of a wet-strength agent.
 20. A paper or nonwoven product of claim 11, wherein the cationic polyelectrolyte is polyaminoamideepichlorhydrin (PAE) and/or glyoxylated polyacryl amide (G-PAM).
 21. A tissue paper of claim 12, wherein the tissue paper has a wet tensile index of at least 7.0% Nm/g.
 22. A tissue paper of claim 12, wherein the tissue paper has a wet tensile index of at least 7.5% Nm/g. 